During the service of a cylinder head, the mechanic is often faced with the question of whether or not to replace the valve guides. These simple pieces operate in a harsh environment (particularly the exhaust guides) and are subject to wear over long periods.
All aluminum cylinder heads use a valve guide of a different (harder wearing) material. Generally this material is either bronze or cast iron, both materials offering reasonable wear properties and price. Note: Most engine builders recommend bronze guides as they have better wear properties compared to their cast iron equivalents. However, bronze guides typically cost four times more than the cast iron items ($4 compared to $16, for example).
Before the valve guides are replaced, the mechanic must thoroughly inspect the valves, the guides and the valve seats. To complete a thorough inspection of the various parts, the mechanic must fully disassemble the cylinder head. The disassembly will include removing the valves, (OHC type), plugs, and any seals (note: all seals should automatically be replaced during a cylinder head service).
Supporting the Head
With the head fully disassembled and inspected, the mechanic should prepare an area for the work to be done. As aluminum heads are relatively easy to damage, it is good practice to make a wooden support (see photograph). In addition, drifts of a suitable size must be ready to be used as soon as the head has been warmed (see below). The first drift should be of aluminum (round bar stock of 6061 is best) followed by a steel drift of slightly lesser diameter than that of the outside of the guide. For example, for guides that measure 0.500” O/D (outside diameter), the mechanic should use a 7/16” (0.4375”) drift for the second drift that will pass through the guide hole.
To remove the valve guides it is first necessary to heat the cylinder head. The aluminum head will expand approximately twice as fast as a cast iron valve guide, therefore, although the head and guide may be heated at the same time, the guide will effectively loosen as the head warms. The temperature required to heat the head sufficiently, for valve guide removal, is approximately 200 degrees Fahrenheit; however, this temperature is the head temperature not the oven temperature. Therefore the mechanic must check the head temperature periodically to ascertain when the head is at 200 degrees F.
With the head warmed to the specified temperature, the mechanic should place it onto a wooden support. The aluminum drift should be used first to initiate removal of the guide—a good hard hit with a two pound hammer will accomplish this. As the guide moves out through the head, the mechanic should swap the aluminum drift for the steel item to complete the removal. Generally, the mechanic should be able to remove four valve guides (working quickly) without having to re-heat the head.
After the guides have been removed, the holes in the head must be thoroughly cleaned; they should not, however, be opened up by abrasives or drills etc. A simple circular brush in an electric drill used with brake cleaner will polish the hole ready for fitting the new guide.
The head will need to be re-heated before the new guides are fitted and the guides themselves should be placed in a zip-lock bag then put into a freezer (freezing for one hour will be sufficient to shrink the guide slightly which will ease the refitting process).
When the head and guides are the appropriate temperatures, the mechanic should drift the new guides into the head using an aluminum drift. This drift should have a large enough hole in it to slide over the guide—this will ensure the guide is straight and well supported.
Once the new guides have been fitted, the mechanic should re-grind the valves in to ensure a good seal.
Note: Should the valve seats be in need of replacement, the mechanic should entrust the work to an automotive machine shop that will have the necessary machines and tooling. If the head does require new valve seats, the mechanic is advised to have the valve guides replaced by the machine shop at the same time.